It is common to compression mold plastic articles including closures, as contrasted to injection molding plastic articles. Typical patents comprise U.S. Pat. Nos. 2,072,536, 2,155,316, 2,218,456, 2,402,462, 2,891,281, 3,210,805, 4,296,061, 4,314,799, 4,343,754, 4,355,759, 4,497,765, 4,640,673, 4,755,125 and EPO 0 091 653 A2.
In the compression molding of plastic articles, there are inherent variations that can affect the resultant articles. One such variance is the manufacturing tolerance applied to the tools. Accordingly, the molding sets on a machine are not identical. Thus, when the tools are made up in the molding position, the volume of the space between the molding surfaces varies between mold sets. A further variance is the weight and/or volume of the plastic charge that is placed within each mold set.
Among the objectives of the present invention are to provide a method and apparatus for compression molding plastic articles including closures wherein the forming pressure can be accurately controlled; wherein the forming pressure can be readily adjusted; wherein lateral forces on the tooling are not applied directly to the forming tool; wherein the tooling may be readily replaced; wherein the number and size of tool stations can be readily changed; and wherein various kinds and sizes of articles including closures can be readily made by changing the tooling and associated actuating mechanisms; wherein the tooling will compensate for variations in pellet or charge weight, variations in mold tooling volume in the closed mold position and wherein a substantial overload such as a double charge of plastic can be readily absorbed without overloading the tooling or the overall apparatus.
In accordance with the invention, a method and apparatus for compression molding plastic articles including closures includes providing coacting sets of tools including a first set for moving a core and core sleeve into engagement with a cavity mold relative to a second set of tooling. The first set of tooling includes an actuator between the tooling and a fixed upper cam. The second set of tooling includes an actuator supporting the cavity mold and associated with a lower fixed cam. A gas cylinder charged with atmospheric gas at a predetermined pressure, preferably nitrogen, is provided in the second set of tooling and controls the compression molding force. In a preferred form, a plurality of sets of tooling are provided in circumferentially spaced relation on a rotating turret supported by a central column. A common manifold supplies the pressure at accurately controlled pressure to each of the nitrogen cylinders.
In accordance with the invention, each individual gas charged cylinder in association with its individual tooling has a common predetermined pressure as defined by the system pressure, each cylinder being connected in parallel with each of all the tooling cylinders by means of connecting pipework and manifolds.
In the preferred form, each of the gas cylinders has a common diameter and consequently will require an equal force to deflect each of the pistons within its respective cylinder against the common system pressure. In the compression molding process, it is preferred that each molded part be molded with a constant molding force and that variations are held to a minimum. This force will be sufficient to fully form the part but not so high as to cause flashing of the material from the adjoining mold surfaces.
It is well known in the art that volumetric variations occur within the tooling due to manufacturing tolerances and that the plastic material charge can vary in weight and volume. Further, it is understood that occasional gross errors will occur in the pellet charge volume as a result of incorrect machine setting, or as a result of double charge of plastic, or as a result of a prior molded part not being released from the mold prior to receiving a further charge of material.
For this reason, it is preferred to include a means to compensate for the accumulated volumetric variance resulting from the above phenomena while not exceeding the preset molding force established by the system pressure previously mentioned.
Further, there is a limit of volumetric variance beyond which the resulting part is nonfunctional as defined by the desired geometry of the part being molded.
In a preferred form, each of the first set of tools is advanced a common fixed stroke toward the respective cavity within the second set of tools to close the mold and to form the plastic to fill the mold. Normally the plastic charge is within the desired weight tolerance to correctly form the part and the pressure in the melt builds up during the forming stage until the desired molding force is reached and the cavity is deflected against the supporting gas cylinder to limit any further increase in molding force. By this means, when the desired molding force is reached, the volumetric space within the tooling combination will remain constant for the duration of the stroke of the first set of tooling.
If the plastic charge was of a size less than the lower tolerance to produce a functional part and of insufficient size to fill the mold, then the desired molding force would not be achieved prior to the completion of the stroke of the first tooling and the resulting part could have voids, often referred to as "shorts".
Clearly, if a charge of material exceeded the maximum tolerance to produce a quality part, then it is important to ensure that the tooling volume is compensated by further deflection of the cavity under the control of its respective gas cylinder in order that the mold force does not reach such proportion as to damage the tooling or the machine.
To maintain a constant molding force within each of successive tools, it is preferred that the force compensating means is contained within the tool and thus independent of any external influences and has a minimum of moving parts to reduce friction. It is also preferred not to use mechanical springs for force compensation as they have a variance in spring rating, are subject to failure, and cannot readily be re-set.